Vaccination

Antigen preparations used in vaccines

A wide variety of preparations are used as vaccines (Fig. 18.1). In general, the more antigens of the microbe retained in the vaccine, the better, and living organisms tend to be more effective than killed organisms. Exceptions to this rule are:

Fig. 18.1 The main antigenic preparations

A wide range of antigenic preparations are used as vaccines.

Live vaccines can be natural or attenuated organisms

Natural live vaccines have rarely been used

Apart from vaccinia, no other completely natural organism has ever come into standard use. However:

• bovine and simian rotaviruses have been tried in children;

• the vole tubercle bacillus was once popular against tuberculosis, and

• in the Middle East and Russia Leishmania infection from mild cases is reputed to induce immunity.

Although it is possible that another good heterologous vaccine will be found, safety problems with this approach remain considerable. Nevertheless, the ability to genetically manipulate heterologous organisms can increase safety (for example, by removing genes responsible for virulence) and allow the creation of hybrid organisms capable of eliciting a strong immune response to the natural human pathogen. A recent example of the successful application of this approach is in the development of new rotavirus vaccines.

The new rotavirus vaccines should prevent many infants from dying in developing countries

Infantile diarrhea caused by rotavirus (a double stranded RNA virus of the Reoviridae family) infection accounts for approximately 600 000 deaths per annum worldwide, with the majority of these occurring in developing countries. An effective vaccine would therefore be of great value and save many lives.

In 1998 a vaccine based on live rotavirus was tested on large numbers of infants in the USA. Although the vaccine was found to be effective, approximately 1 in 2500 vaccinated infants developed intussusception (a potentially fatal bowel condition) and it was withdrawn by the manufacturer.

Greater knowledge of the viral molecular biology and techniques manipulating the virus in vitro, have now led to the development of two new and highly effective vaccines against rotavirus infection. These vaccines are marketed under the names Rotarix™ and RotaTeq™. The former is a live attenuated virus produced by repeated passage in animal cell lines in the laboratory (see below). The latter is a complex of 5 different hybrid viruses providing immunity to the 5 most prevalent viral strains. These viruses are based on the bovine rotavirus which is naturally attenuated in human hosts. Into this backbone have been incorporated the human rotavirus viral capsid proteins VP4 or VP7, which are known to be the targets of natural immunity in human rotavirus infection (Fig. 18.w1).

Fig. 18.w1 Reassortment to generate an oral live rotavirus vaccine

Rotavirus reassortant to generate oral live virus vaccine. The human chromosome in the reassorted virus which codes for the capsid protein is colored green. The bovine chromosome coding for the spike protein is brown. RotaTeq is a polyvalent vaccine consisting of five human-bovine reassortants: four G serotypes (G1, G2, G3, G4) representing 80% of the G strains circulating worldwide and one P serotype representing >75% of the P strains circulating worldwide.

(Reprinted by permission from Macmillan Publishers Ltd: Nature Medicine (Buckland BC: The process development challenge for a new vaccine. Nature Medicine 2005;11:S16–S19.).

The vaccine has proven to be safe and well tolerated in testing in Europe and the USA and provides high levels of protection against rotavirus gastroenteritis. It is hoped that future experience will prove it to be equally effective in developing countries.

Attenuated live vaccines have been highly successful

Historically, the preferred strategy for vaccine development has been to attenuate a human pathogen, with the aim of diminishing its virulence while retaining the desired antigens.

This was first done successfully by Calmette and Guérin with a bovine strain (Mycobacterium bovis) of Mycobacterium tuberculosis, which during 13 years (1908–1921) of culture in vitro changed to the much less virulent form now known as BCG (bacille Calmette–Guérin), which has at least some protective effect against tuberculosis.

The real successes were with viruses, starting with the 17D strain of yellow fever virus obtained by passage in mice and chicken embryos (1937), and followed by a roughly similar approach with polio, measles, mumps, and rubella (Fig. 18.2)

Fig. 18.2 Live attenuated vaccines

Attenuated vaccines are available for many, but not all, infections. In general it has proved easier to attenuate viruses than bacteria.

Q. Why would passage of a virus in a non-human species be a rational way of developing a vaccine for use in humans?

A. The selective pressure on the virus to retain genes needed for virulence in humans and for human-to-human transfer is removed. Consequently some variants may lose these genes and are at no disadvantage in the animal, but will retain antigenicity for use as attenuated vaccine strains.

Just how successful the vaccines for polio, measles, mumps, and rubella are is shown by the decline in these four diseases between 1950 and 1980 (Fig. 18.3).

Fig. 18.3 Effect of vaccination on the incidence of viral disease

The effect of vaccination on the incidence of various viral diseases in the USA has been that most infections have shown a dramatic downward trend since the introduction of a vaccine (arrows).

Attenuated microorganisms are less able to cause disease in their natural host

Attenuation ‘changes’ microorganisms to make them less able to grow and cause disease in their natural host. In early attenuated organisms, ‘changed’ meant a purely random set of mutations induced by adverse conditions of growth. Vaccine candidates were selected by constantly monitoring for retention of antigenicity and loss of virulence – a tedious process.

When viral gene sequencing became possible it emerged that the results of attenuation were widely divergent. An example is the divergence between the three types of live (Sabin) polio vaccine:

• type 1 polio has 57 mutations and has almost never reverted to wild type;

• types 2 and 3 vaccines depend for their safety and virulence on only two key mutations – frequent reversion to wild type has occurred, in some cases leading to outbreaks of paralytic poliomyelitis.

Those genes not essential for replication of the virus are mostly concerned with evasion of host responses and virulence, that is the ability to replicate efficiently and disseminate widely within the body, with pathological consequences. Many pathogenic viruses contain virulence genes that mimic or interfere with cytokine and chemokine function. Some of these have sequence homology to their mammalian counterparts and others do not.

Q. It is often found that attenuated viruses that are avirulent do not make good vaccines. Why should this be?

A. Inflammation induced by damage to the host has an adjuvant effect, leading to more effective presentation of the vaccine antigens.

Killed vaccines are intact but non-living organisms

Killed vaccines are the successors of Pasteur’s killed vaccines mentioned above:

• some are very effective (rabies and the Salk polio vaccine);

• some are moderately effective (typhoid, cholera, and influenza);

• some are of debatable value (plague and typhus); and

• some are controversial on the grounds of toxicity (pertussis).

Figure 18.4 lists the main killed vaccines in use today. These are gradually being replaced by attenuated or subunit vaccines. However, in the case of polio, some countries are reverting to the use of killed vaccine which is safer than the attenuated vaccine, even though it is less effective. This choice only becomes relevant when the risk of contracting the disease is low in comparison with the risk of developing adverse reactions to the vaccine.

Fig. 18.4 Killed (whole organism) vaccines

The principal vaccines using killed whole organisms.

Inactivated toxins and toxoids are the most successful bacterial vaccines

The most successful of all bacterial vaccines – tetanus and diphtheria – are based on inactivated exotoxins (Fig. 18.5), and in principle the same approach can be used for several other infections. An inactive, mutant form of diphtheria toxin (CRM₁₉₇) has been used as the basis for a number of newly generated conjugate vaccines (see below).

Fig. 18.5 Toxin-based vaccines

The principal toxin-based vaccines. Note that there are no vaccines against the numerous staphylococcal and streptococcal exotoxins, or against bacterial endotoxins such as lipopolysaccharides.

Subunit vaccines and carriers

Aside from the toxin-based vaccines, which are subunits of their respective microorganisms, a number of other vaccines are in use which employ antigens either purified from microorganisms or produced by recombinant DNA technology (Fig. 18.6). For example, a recombinant Hepatitis B surface antigen synthesized in baker’s yeast, has been in use since 1986.

Fig. 18.6 Subunit vaccines

Conjugate vaccines are replacing pure polysaccharides. N. meningitidis type B is non-immunogenic in humans because the capsular polysaccharide cross-reacts with self carbohydrates towards which the host is immunologically tolerant.

Acellular pertussis vaccine consisting of a small number of proteins purified from the bacterium has been available for some years now, and has been shown to be effective, safer and less toxic than the whole killed-organism vaccine. It is usually administered as part of a DTaP (Diphtheria, Tetanus, Pertussis) combination vaccine routinely given to infants.

Conjugate vaccines are effective at inducing antibodies to carbohydrate antigens

Although protein antigens such as hepatitis B surface antigen are immunogenic when given with alum adjuvant (see below), for many types of bacteria, virulence is determined by the bacterial capsular polysaccharide, prime examples being Neisseria meningitidis, Streptococcus pneumoniae, and Haemophilus influenzae B. Such carbohydrate antigens, though they can be isolated and have been used for vaccination, are poorly immunogenic, particularly in infants under 2 years, and often do not induce IgG responses or long-lasting protection. Attempts to boost immunity by repeat administration of these vaccines can actually compromise immunity by depleting the pool of antibody-producing B cells.

Q. Why do polysaccharide antigens not induce IgG responses or lasting immunity?

A. Polysaccharide antigens are not processed for presentation to TH cells, so they do not induce class switching, affinity maturation, or generate memory T cells (see Chapters 8 and 9).

A major advance in the efficacy of subunit vaccines has been obtained by conjugating the purified polysaccharides to carrier proteins such as tetanus or diphtheria toxoid. These protein carriers, which can now be produced in highly purified form by recombinant DNA techniques, are presumed to recruit TH cells and the conjugates induce IgG antibody responses and more effective long lasting protection.

Starting with Haemophilus influenzae (Hib) in the early 1990s, conjugate vaccines for Neisseria meningitis strains A, C, Y and W-135 are also now in widespread usage. In the UK up until 1992 when the vaccine was introduced, Hib was the major cause of infantile meningitis leading to many hundreds of cases per year. The introduction of the vaccine led to a very rapid decline making Hib meningitis now a very rare occurrence (Fig. 18.7).

Fig. 18.7 Introduction of the vaccine led to a rapid decline in Hib meningitis

Incidence of H. influenzae type B in children in England and Wales (1990–2004). Vaccination was initiated in 1992. There was an upsurge in cases after 1999, possibly because the vaccine was less effective when used in combination with other vaccines, but this was controlled with a further campaign, initiated in 2003.

(Based on data from the Health Protection Agency.)

Conjugate meningitis vaccines

The serogroup C meningococcal vaccine was first introduced in the UK in 1999 and has resulted in a reduction of >80% in the incidence of infection. The vaccine has also been used successfully in Australia, Canada, and in Europe. Since 2005 a tetravalent combination conjugate vaccine active against strains A, C, Y, and W-135 has been available across the developed world.

In many parts of Africa meninigitis is endemic. Across the so-called ‘meningitis belt’ of the sub-Saharan region hundreds of thousands of cases per year may occur during epidemics, mainly of serogroup A and latterly W-135 type meningococci. As mentioned in the main text, a version of the combination conjugate vaccine, labelled MenAfriVac™ and produced in India at reasonable cost especially for the developing world, has just been licensed for a phased roll-out across Africa starting in late 2010.

One major success with the Neiserria vaccines has been the generation of good herd immunity which is important, as in unimmunized populations this organism colonizes many individuals (about 25–30% of healthy young adults can carry the organism nasopharyngeally) and these are the main transmitters of the disease. A seven valent conjugate vaccine against pneumonia (Streptococcus pneumoniae) has been available in the USA since 2000 and in the UK since 2006.

Adjuvants enhance antibody production

The increasing use of purified or recombinant antigens has refocused attention on the requirement to boost immune responses through the use of adjuvants. These are often necessary as the antigens on their own are insufficiently immunogenic.

Work in the 1920s on the production of animal sera for human therapy discovered that certain substances, notably aluminum salts (alum), added to or emulsified with an antigen, greatly enhance antibody production – that is, they act as adjuvants. Aluminum hydroxide is still widely used with, for example, diphtheria and tetanus toxoids.

The difficulty with adjuvants is that they mediate their effect through stimulating the inflammatory response, generally necessary to produce a good immune response to antigen. Unfortunately, the inflammatory response is often responsible for the side effects of immunization, such as pain and swelling at the injection site, and can lead to greater malaise, elevated temperature and/or ‘flu-like symptoms’. These are often impediments to vaccine uptake, especially when the distress is caused in small infants who are the commonest vaccine recipients.

With modern understanding of the processes leading to lymphocyte triggering and the development of memory, it is hoped that better adjuvants can be developed. Considerable efforts have been made to produce better adjuvants, particularly for T cell-mediated responses. Figure 18.8 gives a list of these, most of which are still highly experimental or too toxic for use in humans. One recent exception however, is monophosphoryl lipid A (MPL). This compound is derived from chemical degradation of lipopolysaccharide (LPS), the major component of the cell wall of gram negative bacteria. MPL retains potent adjuvant activity but, unlike LPS itself, has low toxicity. It is combined with alum in an adjuvant product AS04, which is incorporated in the new HPV vaccine, Cervarix™. In another adjuvant formulation (AS02) MPL is combined with a saponin (QS21) in an oil-water emulsion. AS02 has proven to be of major importance in boosting the immunogenicity of the candidate malaria vaccine RS,S.

Fig. 18.8 Adjuvants

A variety of foreign and endogenous substances can act as adjuvants, but only aluminum and calcium salts and pertussis are routinely used in clinical practice.

Vaccine administration

Most vaccines are delivered by injection

Administration by injection presents some risks, particularly in developing countries, where re-use of needles and syringes may transmit disease, particularly HIV. Alternatives to needle delivery do exist, however, and can be beneficial for use in mass vaccination programs and for improving compliance in those with ‘needle phobia’. Mass vaccination, for many years, made use of multiuse jet injectors that fire a high-velocity liquid stream, which is very effective. Unfortunately, the possibility of cross-contamination from the reusable design has, in more recent years, limited their application. Efforts are now being made to develop disposable single use cartridges for such injectors, but inevitably at greater cost per vaccination.

Jet injectors can deliver vaccine intramuscularly, as with a needle, but they can also be used for cutaneous delivery, which should help to reduce the discomfort and potential for distress in infants. Cutaneous delivery is a highly effective method for vaccination; the skin harbors many Langerhans’ cells, which are very active in antigen presentation to T cells in lymph nodes, to which they migrate when activated by exposure to antigen. They also help to initiate an inflammatory response through release of cytokines and chemical mediators, all of which can potentiate the vaccine.

The main difficulty with cutaneous delivery is penetrating below the outer, cornified layer of the skin. Techniques to improve this such as the uses of microneedle arrays (Fig. 18.9) are under development and may one day allow vaccination using skin patches, similar to those used currently for delivering (small molecule) drugs, such as contraceptives.

Fig. 18.9 Microneedle array

A, The size of an experimental microneedle array is shown. B, Scanning electron photomicrograph of a microprojection needle array which is coated with vaccine suspension and used to deliver antigen to the skin subcutaneously. A 25-gauge needle is shown (at right) for size comparison.

(Figure redrawn courtesy of J. Matriano (ALZA Corporation) with kind permission of Springer Science and Business Media).

Vaccine efficacy and safety

To be introduced and approved, a vaccine must obviously be effective, and the efficacy of all vaccines is reviewed from time to time. Many factors affect it.

An effective vaccine must induce the right sort of immunity:

Where the ideal type of response is not clear (as in malaria, for instance), designing an effective vaccine becomes correspondingly more difficult. An effective vaccine must also:

Live vaccines are generally more effective than killed vaccines.

Induction of appropriate immunity depends on the properties of the antigen

Living vaccines have the great advantage of providing an increasing antigenic challenge that lasts days or weeks, and inducing it in the right site – which in practice is most important where mucosal immunity is concerned (Fig. 18.10).

Fig. 18.10 Antibody responses to live and killed polio vaccine

The antibody response to orally administered live attenuated polio vaccine (solid lines) and intramuscularly administered killed polio vaccine (broken lines). The live vaccine induces the production of secretory IgA (slgA) in addition to serum antibodies, whereas the killed vaccine induces no nasal or duodenal sIgA. As slgA is the immunoglobulin of the mucosa-associated lymphoid tissue (MALT) system (see Chapter 2), the live vaccine confers protection at the portal of entry of the virus, the gastrointestinal mucosa.

(Courtesy of Professor JR Pattison, Ch. 26 in Brostoff J, et al., eds. Clinical immunology. London: Mosby; 1991.)

Live vaccines are likely to contain the greatest number of microbial antigens, but safety is an issue in a time of increasing concern about the side effects of vaccines.

Vaccines made from whole killed organisms have been used, but because a killed organism no longer has the advantage of producing a prolonged antigenic stimulus, killed vaccines have been frequently replaced by subunit vaccines. These can be associated with several problems:

• purified subunits may be relatively poorly immunogenic and require adjuvants;

• the smaller the antigen, the more histocompatibility complex (MHC) restriction may be a problem (see Chapters 5 and 8); and

• purified polysaccharides are typically thymus independent – they do not bind to MHC and therefore do not immunize T cells.

These problems have been overcome in vaccines that are routinely used in humans by the use of adjuvants and by coupling polysaccharides either to:

• a standard protein carrier such as tetanus or diphtheria toxoid; or

• to a protein from the immunizing organism such as the outer membrane protein of pneumococci.

However, immunization even with the newer conjugate vaccines, especially when used in infants under 12 months, has shown that antibody levels wane after a number of years. For Hib this has indicated that a booster is required, generally given at around school-age, though ironically the tendency for meningococci to colonize healthy individuals can provide a boost to immunity in those who are immunized. This latter effect may become less common in the future with improved herd immunity, and therefore, a reduced frequency of carriage.

MHC restriction is probably more of a hypothetical than real difficulty because most candidate vaccines are large enough to contain several MHC-binding epitopes. Nevertheless, even the most effective vaccines often fail to immunize every individual – for example, about 5% of individuals fail to seroconvert after the full course of hepatitis B vaccine.

Most of the vaccines in routine use in humans depend on the induction of protective antibody. However, for many important infections, particularly of intracellular organisms (e.g. tuberculosis, malaria, and HIV infection), cellular immune responses are important protective mechanisms.

In recent years there has therefore been much effort to develop vaccines to induce immunity of both CD4 and CD8 T cells. So far the use of DNA and viral vectors have been the routes most commonly explored because both of these strategies lead to the production of antigens within cells and therefore the display of processed peptide epitopes on MHC molecules.

Although these methods, particularly combined in prime-boost regimens, have been highly effective in experimental animal models, so far in humans it has proved difficult to induce high frequencies of long-lasting antigen-specific T memory cells.

Even in experimental animals the duration of protection may not be long-lasting, perhaps because protection by cellular mechanisms requires activated effector cells rather than resting memory cells. Such cells are not well maintained in the absence of antigen.

Vaccine safety is an overriding consideration

Vaccine safety is of course a relative term, with minor local pain or swelling at the injection site, and even mild fever, being generally acceptable. More serious complications may stem from the vaccine or from the patient (Fig. 18.11):

• vaccines may be contaminated with unwanted proteins or toxins, or even live viruses;

• supposedly killed vaccines may not have been properly killed;

• attenuated vaccines may revert to the wild type; and

• the patient may be hypersensitive to minute amounts of contaminating protein, or immunocompromised, in which case any living vaccine is usually contraindicated.

Fig. 18.11 Safety problems with vaccine

The potential safety problems encountered with vaccines emphasize the need for continuous monitoring of both production and administration.

Although serious complications are very rare, vaccine safety has now become an overriding consideration, in part because of the very success of vaccines:

• because many childhood infectious diseases have become uncommon in developed countries, the populations of these countries are no longer aware of the potentially devastating effects of infectious diseases;

• unlike most drugs, vaccinations are given to people who have previously been perfectly well;

• the public is becoming increasingly aware of the possibilities of profitable litigation and companies correspondingly more defensive.

Q. Why is vaccine safety perceived as a less important issue when a vaccine is first introduced?

A. At the time of vaccine introduction the prevalence and danger associated with the infection is so great that any risks associated with the vaccine are relatively small.

MMR controversy resulted in measles epidemics

Anti-vaccine movements in the UK are essentially as old as vaccination itself, dating back to a few years after the introduction of smallpox vaccination by Jenner in 1796. In the modern era, difficulties concerning vaccine safety are well illustrated by the controversy over MMR (measles, mumps, and rubella triple vaccine).

In 1998 a paper was published that received wide publicity in the UK media, purporting to support an association between MMR vaccination and the development of autism and chronic bowel disease. Although a large amount of subsequent work failed to substantiate these findings and the original paper has now been retracted, take-up of MMR in the UK and Ireland declined over several years and epidemics of measles occurred because of declining herd immunity (Fig. 18.12).

Fig. 18.12 The MMR controversy resulted in measles epidemics

The fall in uptake of the MMR vaccine in the UK to a low point in 2002, resulted in an upsurge in cases of measles.

(Data from BMJ 2008; 336:729.)

In 2004, the introduction of a new five-valent vaccine containing diphtheria and tetanus toxoids, acellular pertussis, Haemophilus influenzae type b, and inactivated polio virus threatened to result in decreased take-up in the UK, though the vaccine was shown to be safe and effective. It was argued that giving five immunogens simultaneously was ‘too much’ for the delicate immune system of infants. This argument is spurious, as most of the vaccines within it are subunits (except for inactivated polio). The whole vaccine therefore actually contains fewer antigens than the live bacteria and other organisms that infants encounter every day. Fortunately, the vaccine take up has remained high.

Vaccines in general use have variable success rates

The vaccines in standard use worldwide are listed in Figure 18.13. Four of them – polio, measles, mumps, and rubella – are so successful that these diseases are earmarked for eradication early in the 21st century. If this happens, it will be an extraordinary achievement, because mathematical modeling suggests that they are all more ‘difficult’ targets for eradication than smallpox was.

Fig. 18.13 Vaccines in general use

Vaccines that are currently given, as far as is possible, to all individuals.

In the case of polio, where reversion to virulence of types 2 and 3 can occur, it has been suggested that it will be necessary to switch to the use of killed virus vaccine for some years, so that virulent virus shed by live virus-vaccinated individuals is no longer produced.

For a number of reasons, other vaccines are less likely to lead to eradication of disease. These include:

One of the future problems is going to be maintaining awareness of the need for vaccination against diseases that seem to be disappearing, while, as the reservoir of infection diminishes, cases tend to occur at a later age, which with measles and rubella could actually lead to worse clinical consequences.

Some vaccines are reserved for special groups only

In the developed world BCG and hepatitis B fall into this category, but some vaccines will probably always be confined to selected populations – travelers, nurses, the elderly, etc. (Fig. 18.14). In some cases this is because of:

• geographical restrictions (e.g. yellow fever);

• the rarity of exposure (e.g. rabies);

• problems in producing sufficient vaccine in time to meet the demand (e.g. each influenza epidemic is caused by a different strain, requiring a new vaccine).

Fig. 18.14 Vaccines restricted to certain groups

Vaccines that are currently restricted to certain groups.

Flu pandemics caused by emergence of totally novel influenza strains occur periodically, often caused by the acquisition of genetic material from strains of flu that normally infect animals, such as equine or avian influenza.

In the recent past, major pandemic scares have surfaced in relation to avian and swine flu. Intensive efforts have been made to improve vaccine production methods such that sufficient vaccine is available to deal with such outbreaks. This has involved production of virus in cell culture rather than, conventionally, in chicken eggs, and the application of new immunogens based on VLPs consisting of recombinant antigen mixtures, not inactivated virus. Virosomes produced from purified, solubilized virus complexed with lipid vesicles have also been employed and possess enhanced immunogenicity compared to conventional vaccines. Virosomal vaccine has proven highly effective in elderly patients.

Both the hemagglutinin and neuraminidase antigens, which together make up the outer layer of the virus and are the antigens of importance in the vaccine, are however, subject to extensive variation. A vaccine effective against all strains of influenza would therefore be of tremendous value. A promising approach to this problem is to use chemical modification, such as glycosylation to reduce the immunodominance of the variable regions of these antigens, such that significant titers of antibody to invariant regions of these proteins may be induced.

Vaccines for parasitic and some other infections are only experimental

Some of the most intensively researched vaccines are those for the major tropical protozoal and worm infections (see Chapter 15). However, none has come into standard use, and some have argued that none will because none of these diseases induces effective immunity and ‘you cannot improve on nature’.

Nevertheless, extensive work in laboratory animals has shown that vaccines against malaria, leishmaniasis, and schistosomiasis are perfectly feasible, and there is a moderately effective vaccine against babesia in dogs. In cattle an irradiated vaccine against lungworm has been in veterinary use for decades.

Q. Why, even with effective vaccines, will it be impossible to eradicate many parasitic diseases?

A. Many parasites have alternative species to humans as their host (see Chapter 15).

It remains possible, however, that the parasitic diseases of humans are significantly more difficult to treat, possibly because of the polymorphic and rapidly changing nature of many parasitic antigens. For example:

• none of the small animal models of malaria shows such extensive antigenic variation as Plasmodium falciparum, the protozoon causing malignant tertian malaria in humans;

• similarly, rats appear to be much easier to immunize against schistosomiasis than other animals, including possibly humans.

Part of the problem is that in the laboratory these parasites are usually not propagated in their natural host.

A vaccine against Plasmodium falciparum, has been one of the most sought after for over a generation, as the burden of disease and death in endemic malarial regions in Africa is huge. Malaria is unusual in that its life cycle offers a variety of possible targets for vaccination (Fig. 18.15). Over the years several trials of clinical malaria vaccine have been published, using antigens derived from either the liver or the blood stage, with only very moderate success.

Fig. 18.15 Malaria vaccine strategies

A number of different approaches to malaria vaccines are being investigated, reflecting the complexity of both the life cycle of malaria and immunity to it.

During the last five years or so, however, a realistic candidate vaccine has emerged from a partnership begun in the early 1980s, between the pharmaceutical company GlaxoSmithKline (GSK) and the US Walter Reed Army Institute of Research (WRAIS). The vaccine known as RS,S is based on a genetically engineered version of the circumsporozoite (CS) protein that is expressed on sporozoites and liver stage schizonts (see Fig. 18.15). Recombinant CS protein is expressed in yeast cells as a fusion protein with the Hepatitis B surface antigen (HBsAg), the basis of the successful recombinant HepB vaccine. Co-expression of the fusion protein along with unmodified HBsAg, allows the formation of VLP-type aggregates of the antigens. To make this preparation sufficiently immunogenic has required combination with the powerful new adjuvant AS02 (see above).

Phase II trials in Africa have shown a significant protective effect on both infection rate and clinical malaria development. A large phase III trial is now underway and, should the initial promise be realized, the first licensed antimalarial vaccine may become available by 2015.

Our understanding of how the RS,S vaccine elicits a protective immune response unfortunately remains poor, and the degree of protection provided is limited, but it is hoped that further research will result in an even more effective second generation vaccine in the future.

A problem with these chronic parasitic diseases is that of immunopathology. For example, the symptoms of Trypanosoma cruzi infection (Chagas’ disease) are largely due to the immune system (i.e. autoimmunity). A bacterial parallel is leprosy, where the symptoms are due to the (apparent) overreactivity of TH1 or TH2 cells. A vaccine that boosted immunity without clearing the pathogen could make these conditions worse.

Another example of this unpleasant possibility is with dengue, where certain antibodies enhance the infection by allowing the virus to enter cells via Fc receptors.

Similarly, a HIV vaccine trial, which though promising in inducing a cell mediated-response, was aborted as the risk of HIV infection in the vaccinated subjects was increased over the unvaccinated controls.

Other viral and bacterial vaccines that are also experimental are:

• attenuated shigella;

• Epstein–Barr virus surface glycoprotein;

• respiratory syncytial virus (RSV);

• group b Streptococcus aureus.

For many diseases there is no vaccine available

No vaccine is currently available for many serious infectious diseases, including staphylococci and streptococci, syphilis, chlamydia, leprosy, and fungal infections. The predominant problem is often the lack of understanding of how to induce effective immunity. HIV infection heads this list of diseases (see Chapter 17), which represent the major challenge for research and development in the coming decade (Fig. 18.w2)

Fig. 18.w2 Major diseases for which no vaccines are available

Diseases for which there is no currently available vaccine.

Passive immunization can be life-saving

Driven from use by the advent of antibiotics, the idea of injecting preformed antibody to treat infection is still valid for certain situations (Fig. 18.16). It can be life-saving when:

Fig. 18.16 Passive immunization

Although not so commonly used as 50 years ago, injections of specific antibody can still be a life-saving treatment in specific clinical conditions.

At the opposite end of the scale, normal pooled human immunoglobulin contains enough antibody against common infections for a dose of 100–400 mg IgG to protect hypogammaglobulinemic patients for a month. Over 1000 donors are used for each pool, and the sera must be screened for HIV and hepatitis B and C.

In this light it is still somewhat surprising that the use of specific monoclonal antibodies, though theoretically highly attractive, has not yet proved to be an improvement on traditional methods, and their chief application to infectious disease at present remains in diagnosis.

One exception to this rule has been the monoclonal antibody Palivizumab™, launched in 1998, which has found application in prophylaxis against respiratory syncytial virus (RSV) infection, where the development of a vaccine has remained elusive. Palivizumab™ has proven effective in protecting high risk individuals, such as premature infants.

Antibody genes can now be engineered to form Fab, single chain Fv, or VH fragments (see Chapter 3). Libraries of these can be expressed in recombinant phages and screened against antigens of interest. Selected antibody fragments can be produced in bulk in bacteria, yeasts, or mammalian cells, for use in vitro or in vivo. This technology has helped in the production of human and humanized mouse monoclonal antibodies for therapeutic application.

Non-specific immunotherapy can boost immune activity

Many of the same compounds that act as adjuvants for vaccines have also been used on their own in an attempt to boost the general level of immune activity (Fig. 18.w3). The best results have been obtained with cytokines, and among these IFNα is the most widely used, mainly for its antiviral properties (but also for certain tumors – see below).

Fig. 18.w3 Non-specific immunotherapy

Non-specific stimulation or inhibition of particular components of the immune system may sometimes be of benefit.

Perhaps the most striking clinical effect of a cytokine has been that of granulocyte colony stimulating factor (G-CSF) in restoring bone marrow function after anti-cancer therapy, with benefit to both bleeding and infection.

Finally cytokine inhibitors can be used for severe or chronic inflammatory conditions. Various ways of inhibiting TNFα and IL-1 have proved valuable:

Modulation of cytokine responses and the use of cytokine inhibitors is now an area of intense investigation and may lead to new immunotherapies in the future.

Immunization against a variety of non-infectious conditions is being investigated

The success of vaccine strategies against infectious disease has sparked renewed interest in the possibility of immunizing against non-communicable diseases, many of which are now the major sources of morbidity and mortality.

The most obvious candidate would be cancers, which are known to sometimes be spontaneously rejected as if they were foreign grafts (see Chapter 22). A large quantity of research is now directed at trying to induce autoimmune rejection of tumors, mainly utilizing genetic modification approaches to increase immunogenicity for the host.

In principle, conception and implantation can be interrupted by inducing immunity against a wide range of pregnancy hormones, of which the most popular has been human chorionic gonadotropin (hCG), the embryo-specific hormone responsible for maintaining the corpus luteum. Alternatively, a number of sperm antigens are potential targets. The outcome of this reseach has thus far been disappointing.

Other uses of immunization that are being explored include:

So far these vaccines are largely experimental, however given the success of anti-TNFα monoclonal antibodies in treating rheumatoid arthritis the possibility of long term treatment by immunization remains an attractive possibility. Current trials in Crohn’s disease, using TNFα coupled to keyhole limpet hemocyanin (KLH), as immunogen, are showing real promise.

Future vaccines

Without doubt the future generation of new, improved and safer vaccines lies in the exploitation of recombinant DNA technology and genetic engineering of pathogenic organisms and their antigens. A development of the ability to clone genes is the possibility of using a benign, non-pathogenic virus as a vector to display antigens to the immune system in a way that mimicks their natural exposure but is without the risks associated with attenuated pathogens. The gene(s) encoding the desired antigen(s) is incorporated into the genetic material of the vector, which can then express the gene, and produce the antigen in situ. The vector can then be injected into the patient and in some cases also allowed to replicate.